Is it better to learn a domain or a skill?

I have two opportunities. One is to learn a domain, software monitoring, the other is to learn a skill, building web services. I don’t know much about either. Which is more useful in the long term for an individual contributor? I think the manager would be better of knowing the domain of software monitoring, but wondering if this is useful for an individual contributor. I’m trying to ask this question in a generic way, so for instance an equivalent question would be, “would it be better to learn a domain like containerization, or a skill like embedded software?”